The present invention relates to electromagnetic interference (xe2x80x9cEMIxe2x80x9d) shields and, more specifically, to an EMI shield manufactured from a foam core laminated by a metallized fabric or other electrically conductive material or covering.
During normal operation, electronic equipment generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can by of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include gaps or seams between adjacent access panels and around doors and connectors, effective shielding is difficult to attain because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.
An area of concern in electronic enclosures such as personal computers and the like, which connect to peripheral equipment, is the zone surrounding electrical connectors and electrical connections, generally referred to as an input/output (xe2x80x9cI/Oxe2x80x9d) panel. Cutouts and other access are provided in a bezel in the enclosure to facilitate connection of cabling which connect a computer processor to a printer, a display, a keyboard, and other related equipment. The connector sockets are typically mounted on an I/O panel back plane of a printed circuit board. As with other gaps in the enclosure, these cutouts are preferably shielded with an EMI shield.
Specialized EMI shields have been developed for use in gaps and around doors to provide a degree of EMI shielding while permitting operation of enclosure doors and access panels and fitting of connectors. To shield EMI effectively, the shield should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the shield is disposed. Conventional metallic shields manufactured from copper doped with beryllium are widely employed for EMI shielding due to their high level of electrical conductivity. Due to inherent electrical resistance in the shield, a portion of the electromagnetic field being shielded induces a current in the shield, requiring that the shield form a part of an electrically conductive path for passing the induced current flow to ground. Failure to ground the shield adequately could result in radiation of an electromagnetic field from a side of the shield opposite the primary EMI field.
In addition to the desirable qualities of high conductivity and grounding capability, EMI shields should be elastically compliant and resilient to compensate for variable gap widths and door operation, yet tough to withstand repeated door closure and connector installation without failing due to metal fatigue. EMI shields should also be configured to ensure intimate electrical contact with proximate structure while presenting minimal force resistance to door closure and connector installation. It is also desirable that the shield be resistant to galvanic corrosion which can occur when dissimilar metals are in contact with each other for extended periods of time. Low cost, ease of manufacture, and ease of installation are also desirable characteristics for achieving broad use and commercial success.
Conventional metallic EMI shields, often referred to as copper beryllium finger strips, include a plurality of cantilevered or bridged fingers that provide spring and wiping actions when compressed. Other types of EMI shields include closed-cell foam sponges having metallic wire mesh knitted thereover or metallized fabric bonded thereto. Metallic wire mesh may also be knitted over silicone tubing. Strips of rolled metallic wire mesh, without foam or tubing inserts, are also employed.
One problem with metallic finger strips is that to ensure a sufficient compression force, the copper finger strips are made from thin stock, for example on the order of about 0.05 mm (0.002 inches) to about 0.15 mm (0.006 inches) in thickness Accordingly, sizing of the finger strip uninstalled height and the width of the gap in which it is installed must be controlled to ensure adequate electrical contact when installed and compressed, yet prevent plastic deformation and resultant failure of the strip due to overcompression of the fingers. To enhance toughness, beryllium is added to the copper to form an alloy; however, the beryllium adds cost. Finger strips are also expensive to manufacture, in part due to the costs associated with procuring and developing tooling for outfitting presses and rolling machines to form the complex contours required. Changes to the design of a finger strip to address production or performance problems require the purchase of new tooling and typically incur development costs associated with establishing a reliable, high yield manufacturing process.
Metallic mesh and mesh covered foam shields avoid many of the installation disadvantages of finger strips; however, they can be relatively costly to produce due to the manufacturing controls required to realize acceptable production yields. Further, due to manufacturing tolerances and the number of apertures or slots required for the connectors on an I/O panel, adequate shielding cannot always be ensured due to unshielded gaps.
An I/O port EMI shield according to the invention overcomes many of the limitations and disadvantages of conventional EMI shields. In one embodiment, a foam core is laminated by and bonded to an electrically conductive covering, such as a metallized fabric. A generally planar stiffener is then bonded to at least one of the core and the fabric. The stiffener may have an aperture formed in the stiffener prior to bonding with the core and/or fabric. Thereafter, one or more apertures may be formed in the shield, for example by a die cutting operation, to accurately size and locate desired connector openings in the shield. A silk screen or other printing operation may be used to mark the shield with indicia, such as the type of connector or cable to be employed with a particular port.
In an alternative embodiment, the foam core may be substantially fully wrapped by the metallized fabric or other electrically conductive covering. The stiffener may then be bonded to the fabric, the apertures formed, and the shield printed. In another embodiment, the foam core may be bonded to the stiffener and the assembly fully wrapped by the metallized fabric. The stiffener may have an aperture formed in the stiffener prior to bonding with the core and/or fabric. In any of these embodiments, one or more adhesive strips or spots may be provided to facilitate installation of the shield into the enclosure.
An EMI shield according to the invention combines effective EMI shielding, due to the compressible resilient nature of the foam core, with flexibility of manufacture to accommodate a wide variety of I/O port applications. In one embodiment, the shield may be manufactured by a continuous process, along a manufacturing line. A metallized fabric or other conductive covering in the form of a roll having an adhesive laminated along one side thereof is passed over a heated plate to thermally activate the adhesive. The fabric is then mated with a foam core dispensed from a roll. By initially adhering the foam to the fabric, stretching of the foam core is minimized.
Next, the foam and fabric are mated with the stiffener and passed through a folding die to wrap the fabric around at least opposite edges of the stiffener. In one embodiment, the stiffener may have an adhesive laminated to the side disposed proximate the foam core. In another embodiment, the stiffener may have an aperture formed prior to attaching the foam and fabric. The entire assembly is then pulled through a heated forming die to complete bonding of the fabric to the stiffener and the stiffener to the foam. The shield may then be passed through a rotary die cutter to form the desired apertures in the shield. The shield may also be passed through a silk screen or other printing process to label the apertures and the shield with suitable indicia, as desired. Alternatively, the stiffener could be pre-printed. Also, an adhesive tape or spotting may be applied to a front or rear face of the shield to facilitate installation in the enclosure.